This invention relates generally to an ink composition, and in particular to a publication gravure ink containing a polyurethane bound colorant.
Publication gravure or rotogravure printing is a process in which an engraved metal cylinder is used to transfer ink to paper. The depth of the engraving determines the amount of ink which is picked up by the rotating cylinder, and in turn, determines the depth of shade on the printed sheet. Engraving the metal cylinder is typically accomplished by etching with acid or by mechanical engraving, to produce slight indentations in the surface of the cylinder called cells. The number of cells may range from 22,000 cells per inch for standard quality printing, to 32,000 cells per inch or more for high quality reproductions. Each individual cell is from 20 to 200 microns in width.
The inks used in publication gravure consists of a colorant and a liquid vehicle, in which the colorant, typically a pigment, is dispersed. After the ink is applied, a binder present in the vehicle affixes the colorant to the substrate on which it is printed. The binder may be a drying oil, which oxidizes and is converted from a liquid to a solid film, or a resin dissolved or dispersed in a solvent, which forms a solid film binding the colorant to a substrate after the solvent is evaporated. Combinations of drying oils, resins and organic solvents are also employed.
Gravure inks consist of approximately 30 wt. % of non-volatile components, made up primarily of the binder and colorant. The binder and colorant are typically present in a ratio ranging from about 5:1 to about 1:1 by weight, respectively.
The resolution of gravure printing may be improved by increasing the number of cells per inch. However, as the density of the cells increase, it becomes necessary to make them smaller, which in turn limits the volume of ink which can be picked up and transferred to the paper. Consequently, it becomes increasingly difficult to print darker shades at higher resolutions using the gravure process. Adjusting the color strength, by increasing the solids content of the ink, can raise the viscosity of the ink above acceptable levels.
Publication gravure printing, as well as lithographic printing and flexographic printing, require inks with unique performance characteristics. Pigments, and to a much lesser extent dyes, are employed as colorants in the inks. Typically, the pigments are milled to a mean particle size of less than 2 microns and dispersed in the vehicle, often with the aid of specialized dispersants. It has been proposed that colored polymeric resins, such as polyurethane bound colorants, may be substituted for conventional pigments. Meek, U.S. Pat. No. 3,278,486, suggest that colored polymers can be employed in inks, paints and similar materials. Krutak et al, U.S. Pat. No. 5,194,463 disclose polyurethane polymers having difunctional chromophores copolymerized therewith. The colored polymers can be ground to very fine powders and dispersed in inks or coating compositions. Moore et al., U.S. Pat. No. 5,176,745 disclose poly(oxyalkylene) substituted colorants reacted with alkenyl succinic anhydrides to provide a water soluble colorant for inks. Despite the availability of such prior art colored polymers, they have not been commercially significant in the field of printing inks, especially not in publication gravure printing.
Polyurethane-bound poly(oxyalkylene) substituted colorants are disclosed in Cross et al., U.S. Pat. No. 4,284,729. The colorants are particularly useful for in situ coloration of polyurethane foam.